1. Field of the Invention
The present invention relates to a spark plug having an auxiliary gap in addition to a regular gap defined by a center electrode and a ground electrode in order to effectively suppress flashover which would otherwise propagate from a tip end surface of an insulator of the spark plug toward the interior of the spark plug over the surface of the insulator upon application of high voltage thereto. The present invention effectively suppresses flashover even when used in an engine which is highly likely to suffer smolder, particularly an engine designed such that during a compression stroke fuel is injected in order to form a rich mixture in the vicinity of a spark portion of a spark plug, thereby causing combustion by the spark plug (hereinafter referred to as a "stratified charge combustion engine").
2. Description of the Related Art
Conventionally, a certain spark plug used in an internal combustion engine has not only a main gap defined by a center electrode and a ground electrode--the center electrode being held by an insulator which tapers off toward the tip end thereof and the ground electrode being connected to the tip end of a metallic shell which fixedly supports the insulator--but also an auxiliary gap defined by the center electrode and a projecting inner edge formed at an open end portion of the metallic shell. In such a spark plug, the inner wall surface of the metallic shell--which fixedly supports the insulator and, in turn, holds the center electrode--has substantially the same diameter along the center axis of the center electrode held by the insulator, i.e., is substantially perpendicular to the open end portion of the metallic shell.
In the aforementioned conventional spark plug used in an internal combustion engine, a space (hereinafter may be referred to as a gas volume) defined by the surface of a leg portion of the insulator secured in the metallic shell and the corresponding inner wall surface of the metallic shell mainly extending from a terraced portion toward a tip end of the metallic shell widens toward the tip end. In a combustion chamber of an internal combustion engine, combustion gas generated as a result of ignition of an air-fuel mixture tends to enter the gas volume. Accordingly, during high speed operation, high temperature combustion gas enters the gas volume. As a result, the leg portion of the insulator secured in the metallic shell is subjected to a very severe thermal load, potentially resulting in deterioration in heat resistance of the leg portion.
If, in order to prevent deterioration in heat resistance of the leg portion of the insulator, the gas volume is decreased while the inner wall surface of the metallic shell is maintained at substantially the same diameter along the center axis of the center electrode, a gap between the surface of the insulator and the inner wall surface of the metallic shell becomes significantly narrow at a deep interior portion of the metallic shell; for example, at the terraced portion. Thus, particularly when the engine is not warmed up, such as at start-up, carbon which is generated in association with incomplete combustion of rich mixture enters deep into the metallic shell and adheres to and accumulates on the surface of the insulator. Carbon adhering to and accumulating on the insulator surface causes spark leak, which, when contamination with carbon is significant, may impair startability.
Particularly, in the case of an engine (hereinafter may be referred to as a cylinder-injection-of-fuel engine) in which fuel is directly injected into a combustion chamber, smolder tends to occur with resultant spark leak.
Specifically, a feature of the cylinder-injection-of-fuel engine is employment of a "stratified charge combustion scheme," in which fuel is injected during a compression stroke in order to form a rich mixture in the vicinity of a spark portion of a spark plug, thereby causing combustion by the spark plug. This feature allows leaner overall air/fuel mixtures within the combustion chamber, thereby decreasing fuel consumption. In stratified charge combustion, timing of fuel injection is set to near a predetermined ignition point during the compression stroke so as to initiate combustion through ignition of the rich mixture around a spark plug. Such stratified charge combustion is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) Nos. 41-183922 and 58-178835. According to the disclosed stratified charge combustion, the mixture to be formed around the spark portion of a spark plug is set to a very rich level. Moreover, since combustion temperature does not increase sufficiently, the spark plug suffers smolder due to contamination with carbon.
Meanwhile, even when carbon adheres to the insulator of the spark plug, progress of contamination with the adhering carbon is prevented through burning or blowoff of the carbon effected by spark cleaning action, thereby suppressing reduction in insulating resistance (M.OMEGA.) between the center electrode and the metallic shell.
However, in the case of stratified charge combustion as in a cylinder-injection-of-fuel engine, spark cleaning action fails to follow the progress of contamination with carbon; consequently, contamination with carbon progresses. Progressive contamination with carbon causes the insulating resistance (M.OMEGA.) to decrease, causing increased tendency toward flashover with resultant poor engine operation (engine stall, poor idling condition, poor drivability and defective startup).